implement initial Hyperv keyboard synthetic driver

[honzhan_microsoft.com]

Summary

This patch implements synthetic keyboard driver for FreeBSD on Hyper-V Gen2. In Gen2 mode, the legacy keyboard does not work.

[sepherosa_gmail.com]

Looks pretty good to me. Some minor code style suggestion.

sys/dev/hyperv/input/hv_kbdc.c

68–70

Better use C style comment, or '#if 0 ... #endif'

108–119

May be we should use more consistent style of comment?

/*
 *
 */

or your preference:

/**
 *
 */

308

I'd prefer for (;;)

332–343

We don't indent switch-case:

switch () {
case XXX:
    aaaa;
case YYY:
    bbbb;
default:
    cccc;
}